Many applications require signal measurement in environments with high ambient noise levels. For example, telescopes which determine the temperature of objects at great distances typically measure small amounts infrared energy in high ambient noise environments. An effective method of filtering a signal in low signal to noise ratio environments is by modulating the measured signal and passing it to a locking amplifier. The measured signal can then be isolated from the ambient noise using synchronous detection techniques.
Optical shutters, also referred to as choppers, are used to modulate optical beams or particle streams. By modulating optical beams or particle streams, optical shutters allow these signals to be measured accurately in environments with high ambient noise.
One type of optical shutter developed to modulate optical beams or particle streams uses a disk with a predetermined pattern of apertures and opaque portions. A motor is used to rotate the disk so that a beam passing through the apertures is interrupted at a desired modulation frequency. Shutters utilizing a rotating disk, however, have limitations which are important to overcome.
Many applications take a longer time to make optical/particle measurements. These applications require the rotating disk shutter to rotate at low speeds. One way to operate a motor at low speeds is to decrease the current to the motor. Motors operating at low current have less torque, so that any intermittent friction within the motor affects the modulation frequency. Changes in the modulating frequency, called xe2x80x9cjitterxe2x80x9d, over a measurement cycle introduce error to the measured signal.
Another source of jitter for rotating disk shutters comes from small differences between the sizes of each aperture on the disk. These differences translate to slight changes in the modulating frequency. Shutter disks which produce very little jitter are often expensive to manufacture.
Finally, some applications require the ability to stop the shutter disk at a point where the beam is passing through an aperture, or an xe2x80x9copenxe2x80x9d position. Conversely, some applications require the ability to stop the shutter disk at an opaque or xe2x80x9cclosedxe2x80x9d position. This feature is difficult to produce in rotating disk shutters.
For the foregoing reasons, there is a need for an inexpensive optical shutter capable of modulating light beams or particle streams at low frequencies. There is also a need for an inexpensive optical shutter which is jitter free. Finally, there is a need for an inexpensive optical shutter that can be positioned at an open or closed position.
The present invention is directed to an optical shutter and driver that satisfies these needs. An optical shutter having features of the invention comprises a frame with an aperture and shutter blade for modulating a light beam or a particle stream passing through the aperture. The top end of the shutter blade comprises a first sector and a second sector, each sector having different optical characteristics. For example, one sector may be cut out and the other sector may be reflective. The bottom end of the shutter blade is coupled to a motor means for pivotally rotating the shutter blade over a limited angular range in a clockwise and counter-clockwise direction such that the first sector is aligned with the aperture when the shutter blade is rotated in one direction and the second sector is aligned with the aperture when the shutter blade is rotated in the other direction.
A driver circuit for the optical shutter having a motor means with a phase one coil set and a phase two coil set comprises a means for supplying current through the phase one coil set to ground, a clock means for providing the operating frequency and phase of the optical shutter, and a amplifier means for providing current through the phase two coil set to ground in response to the clock means.